Method and apparatus for full scale dynamic footing load test

ABSTRACT

A method for determining allowable bearing pressures of a steel footing on variable subsurface materials whether native soils, rock, or manmade construction material is disclosed. The method continuously measures vertical displacement by an optical technique of a dismantable steel footing under the impact of a free falling steel drop weight. The load pulse of the drop weight is measured by means of a load cell.

Advances in site investigation techniques have afforded engineers tools to assess the suitability of shallow foundations designs with regards to the reliability under design load cases and economic considerations. Full scale, static load tests have been performed as part of research studies, but do not enjoy widespread use as acceptance criteria for constructed foundations in the profession. The operational expenses are one factor that accounts for the reluctance of engineering professionals to rely on footing load tests. Dynamic load tests provide benefits similar to a full scale load test. A dynamic test method is described using a specially designed apparatus. The apparatus described consists of an easily dismantable steel footing with associated accessories for safely delivering a dynamic load and measurement of the load and footing response. All of the system components in the apparatus are capable of being reused multiple times at different locations and project sites. The associated electronic instrumentation allows direct measurement of the vertical displacement and load pulse at the precision and accuracy required by the engineering profession.

SUMMARY OF THE INVENTION

The test method consists of the assembly of a steel footing (100), anvil (200), and safety frame (300) at the test location. Electronic instrumentation is fastened at designated points on the footing (100) and within the anvil (200). The steel drop hammer (400) is assembled by addition of steel plates (410) to the drop weight frame (430). Assembly of all steps is performed by hand unaided by any special lifting equipment. The assembled drop weight is lifted above the steel footing (100), within the confines of the safety frame (300), and dropped onto the anvil (200) at a height of up to 1.5 m. The load pulse generated by the free falling drop weight striking the anvil is measured by a load cell (220) embedded within the anvil, and the vertical displacement of the steel footing is measured by a position sensitive detector (140). The acquired signals from the load cell and position sensitive detector are used to generate a plot of load and vertical displacement with time. A qualified geotechnical engineer inspects the plot. In light of other data, such as engineering properties of the subsurface material, the geotechnical engineer will determine allowable performance criteria to assess if the footing displacement is satisfactory under the imposed load.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates the setup of the primary components including the steel footing (100), the anvil (200), the safety frame (300), and the drop weight (400). The primary components consist entirely of steel sections including plate, angle, and channel sections as well as the associated nuts and bolts. The secondary components consist of electronic instrumentation and acquisition devices (140, 150, 220), as well as the quick release hook (410).

FIG. 2 presents a plan view of the footing as well as plan and section views of the steel drop weight.

FIG. 3 presents perpendicular sectional views of the steel footing through the center of the footing. These sectional views clarify the fastening points of the steel members as well as the location of the electronic instrumentation.

DETAILED DESCRIPTION OF THE INVENTION

At the discretion of the project engineer, soil conditions at the intended test location are determined by collection of soils samples by soil boring, test pit, or bucket sample. Further material testing is additionally performed to classify the subsurface material and determine strength/compressibility properties. The steel footing beams, drop hammer plates, and electronic accessories are delivered to the site location. When unassembled, the maximum weight of any single steel member is 150 kg. This allows a crew of four laborers to move pieces around the site location by hand. A boom truck is a convenient vehicle to use as it allows mobilization of steel members to a project site by road and facilitates unloading of the vehicle. The steel members are placed around the intended test location as designated by the project surveyor or engineer. The steel footing and drop weight are assembled by hand. An optional 7t crane will facilitate the work of the labor crew but is not expressly required. The footing and drop weight are fastened together from the independent steel members by a system of nuts and bolts. Steel angle and channel sections are comprised of Grade 235 steel or any mild steel with a minimum yield strength of 235 MPa. Steel plate sections are comprised of Grade 509 steel or any carbon steel with a minimum yield strength of 509 MPa. The load cell is placed within the anvil, and the anvil is fastened to the footing. A slab of plywood or neoprene 1″ thick is optionally placed on top of the anvil within the dimensions of the safety frame to assist distribution of the load from eccentric impacts.

The load cell has a minimum capacity of 80T. The use of a load cell with a capacity larger than the anticipated load compromises the resolution and accuracy of the load measurement but reduces the possibility of deformation of the load cell housing during multiple tests under repeated impacts. The resolution of the load cell is less than 0.1% of the full scale load. The accuracy of the load cell is not larger than ±1% of the full scale load. The analog output of the load cell ranges between 1 to 5 mV/V.

The position sensitive detector (PSD) is fastened to the frame using 6 mm diameter stainless steel bolt. The fastening point is the flange of an H beam spanning the steel footing. A steel nut is welded to the flange to secure the bolt fastened to the PSD. The analog output of the PSD ranges between 1 to 10V. The typical resolution of the PSD ranges between 500 to 800 nm. The red laser is a Class II laser generating a laser pulse at a wavelength of 635 nm and at a power less than 1 mW. Shortpass and longpass optical filters with dielectric hard coatings are fastened using adhesives to the PSD housing to block light illuminating the PSD at wave lengths less than 635 nm and greater than 670 nm. The red laser is mounted on a short tripod at a minimum distance 5 m away from the footing and the beam is aligned to shine on the PSD.

The acquisition system is connected to the PSD and the load cell. The acquisition system for the PSD consists of an analog to digital converter. The acquisition system for the load cell consists of an analog to digital converter using a full bridge strain module. The full bridge strain module is capable of 24 bit resolution and delivery of a suitable excitation voltage on the order of 1V to 10V to the load cell. The acquisition system uses screw terminals to facilitate connection of instrumentation data cables to the acquisition terminals. The converted digital signal is acquired using a computer program on a laptop computer. Prior to commencing the test, the digital signal is checked to ensure that the instruments are responsive, and that measurements will remain within the instrumentation range. Review of the signal ensures the initial load cell reading is a zero load signal. Sampling frequency of both analog signals is set to a minimum 5 kHz.

The acquisition system acquires simultaneous converted digital signals simultaneously by using a single acquisition computer program on the acquisition laptop.

Once the steel footing and drop weight are assembled, a mobile crane of a minimum 7t capacity is mobilized to conduct the test. The crane places the drop hammer on the anvil and within the safety frame. A lineman attaches a safety snap hook to the drop weight and the crane driver lifts the drop weight. The drop weight is placed within the confines of the safety cage and the snap hook is disengaged. The safety frame ensures the drop weight lands on the anvil and does not rebound onto the steel footing. The lineman attaches the release hook to the crane and drop weight. When the drop weight is secured, the crane driver lifts the weight using the quick release hook. The quick release hook will allow the drop weight to drop in free fall onto the anvil. The test arrangement prior to release of the drop weight is illustrated in FIG. 1.

A seating blow at about 5 cm above the anvil is performed. When the drop weight is at the required height, the lineman pulls the release line and the drop weight is released. The drop weight strikes the anvil and remains within a safety frame.

An additional three drop heights are designated by the project engineer. In the absence of such an instruction, the nominal drop heights above the anvil are 0.5 m, 1.0 m, and 1.5 m. Following the completion of the third drop height, the test is concluded. The supervising engineer performs additional blows at varying heights and drops at their discretion. Following conclusion of the test, the equipment is unfastened and loaded into a vehicle for demobilization from the location.

The embodiments of the invention which are claimed are described in the following section. 

I claim:
 1. A test method for measuring vertical deflection of a dismantable steel footing (100) under the load delivered by a free falling steel drop weight (400).
 2. A method according to claim 1 in that vertical displacement is measured optically using a position sensitive photo detector illuminated by a red laser source at a distance in excess of 5 m from the footing perimeter.
 3. A method according to claim 1 in that measurement of load is acquired by an electrical resistance strain gage load cell.
 4. A method according to claim 1 in that measurements of vertical displacement and load are acquired using analog acquisition equipment at a high sample rates in excess of 5 kHz.
 5. A method according to claim 1 in the steel drop weight is released from a quick release hook suspended by mobile crane.
 6. An apparatus consisting of the assembly of steel sections into the steel footing configuration (100), the anvil (200), the safety frame (300), and the steel drop weight (400). 